Moleskine notebooks are a classic design, based on a style of notebook popular in Paris in the 19th and 20th centuries. In modern times, these handy books had disappeared, but Moleskine's co-founder, Maria Sebregondi wanted to bring them back. They're made popular by a simplicity combined with a useful set of features. An elastic strap holds them neatly closed when not in use, keeping them safe in the pocket or bag. A pocket in the back can tuck away receipts, tickets, some paper money, or other little extras. There's a bookmark to keep your place. The covers are simple, but strong enough to protect the notebook in use. And other than that, nothing.
Note that the sizes for Moleskine notebooks are not standard (apart from the pocket notebooks which are a familiar 90mm x 140mm). Extra-small is 65mm x 105mm, a little narrower than A7; medium is 118mm x 180mm; large is 135mm x 210mm, a little narrower than A5; and extra large is 190mm x 250mm.